Sheena-
I love the sound of my own voice but it would be bad pool to just re-cook what has been done so well already. Loren Cordain has addressed the dietary calcium question very well. Check this stuff out:

The Cliff’s notes on this topic are: Calcium intake is not the issue, calcium retention due to a net alkalinizing diet, and the ratio of calcium, magnesium and zinc, these are the important issues regarding bone health and other elements related to calcium.